Field of the Invention
The present invention relates to a method of forming a bitline and a bitline contact to a dynamic memory cell array, and a dynamic memory cell array having at least one bitline and a bitline contact manufactured by the method.
A schematic cross-sectional view of an exemplary single DRAM (Dynamic Random Access Memory) cell is shown in FIG. 4, wherein reference numeral 9 denotes a storage capacitor, reference numeral 10 denotes the source/drain region and reference numeral 18 denotes the gate electrode of a transistor. In FIG. 4, the storage capacitor 9 for storing information in the form of an electrical charge representing a logic value such as 0 or 1 is implemented as a trench capacitor. The trench capacitor is disposed in a trench 16 having a collar 11 preventing formation a parasitic capacitor, and having a surface strap 12 that also can be implemented as a buried strap so as to electrically connect the top electrode of the storage capacitor 9 with the source/drain region 10 of the transistor. A bitline 1 is connected via a bitline contact 2 with the source/drain region 10 of the transistor for reading the signal stored in the storage capacitor. The gate electrode 18 of the transistor is actuated by the wordline, which is not illustrated, to read the stored information from the capacitor 9. In a DRAM cell array and, in particular, a so-called embedded DRAM cell array, additional metalization layers such as the M1 metalization layer 15 are provided.
In the course of further development of DRAM cells and DRAM cell arrays, it has been attempted to increase the storage density of the memory cell array. As a condition of a further increase of the storage density, the area of the single memory cell has to shrink.
FIG. 6 shows the layout of an exemplary memory cell array implementing a so-called 8-F2-cell architecture. The array includes a storage trench capacitor and a planar transistor for each of the memory cells. For each of the memory cells, an area of 8F2 is needed, wherein F denotes the smallest structural length that can be produced in the technology employed. The bitlines 1 are implemented as stripes and are extending parallel to each other, wherein the width as well as the distance between each of the bitlines amount to F, respectively. The word lines 6 each have a width as well as a distance to each other of F, respectively. The word lines 6 are disposed perpendicularly to the bitlines 1. The active areas 13 of each of the memory cells are disposed beneath the bitlines 1, and two wordlines 6 that are crossing each other above each of the active areas 13. The active areas 13 are disposed at staggered positions to each other beneath neighboring bitlines 1. A bitline contact 2 providing an electrical contact between the corresponding bitline 1 and the source/drain region 10 of the active area 13 is disposed in the middle of each of the active areas, respectively. The trenches 16 housing the trench capacitors 9 are disposed beneath the word lines 6. A gate electrode 18 of the corresponding transistor is disposed at the crossing points between one of the bitlines 1 and one of the wordlines 6 within the active areas 13.
The active areas 13 extend between two trenches 16. Each of the active areas 13 includes two transistors, which are connected with the corresponding bitline 1 via a common bitline contact 2. In dependence on the actuated wordline 6, the corresponding storage capacitor 9, which is disposed in one of the two trenches 16, is read.
Usually, in commonly used DRAM cell arrays, the bitlines as well as the bitline contacts are made of tungsten, and they are separated from each other by a dielectric material such as silicon dioxide.
FIGS. 5A and 5B illustrate the bitline and the bitline contact as manufactured by the prior-art dual damascene process. FIG. 5A shows a cross-section in a direction perpendicular to the wordlines 6. FIG. 5B shows a cross-section in a direction parallel to the wordlines 6.
In FIGS. 5A and 5B reference numeral 1 denotes a bitline, which forms part of the so-called M0 metalization layer, whereas reference numeral 2 denotes a bitline contact extending to the source/drain region 10 of the underlying transistor. Between the wordlines 6, a BPSG layer 3 is filled and planarized to provide an electrical insulation, and a first dielectric layer 4. The first dielectric layer 4 is normally silicon dioxide deposited by the TEOS (tetraethylorthosilicate) process. The first dielectric layer 4 is provided to electrically insulate the bitline contacts 2 and the bitlines 1 from each other.
According to the dual damascene process, the first dielectric layer 4 is deposited on the surface of the BPSG layer 3 filling the space between the wordlines 6. Then, a first photoresist material is applied, patterned to define the bitline contacts 2, and, then, the bitline contacts 2 are etched into the first dielectric layer 4. Afterward, the first photoresist material is removed. Next, a second photoresist material is applied and patterned to define the bitlines 1. Then, the bitlines 1 are etched into the first dielectric layer 4. In a following step, the second photoresist material is removed. Finally, the etched portions are filled with tungsten so as to provide the bitline contacts 2 and the bitlines 1 in one single step. Thereafter, a chemical mechanical polishing step is performed to remove the remaining tungsten material from the surface.
In the usually employed standard process of manufacturing DRAM cell arrays, next, a so-called retention anneal process is performed so as to remove crystal defects especially in the capacitor region of the DRAM cell whereby the retention time of the DRAM cell is set and, thus, the functionality of the DRAM cell is ensured. This retention anneal process is usually a furnace anneal process wherein the temperature is ramped up to temperatures higher than approximately 800xc2x0 C.
Afterward, the other metalization layers such as the M1 (metalization 1) and the M2 (metalization 2) layers are deposited by known methods.
One problem associated with the conventionally employed bitlines made of tungsten is that the coupling of neighboring bitlines drastically increases with decreasing distances between them. This bitline coupling extremely reduces the device performance and thus is one of the most critical yield detractors in the shrinkage of DRAM cell size.
Hitherto, efforts have been made to increase the bitline layer thickness while at the same time reducing the line widths of the tungsten bitlines. Alternatively, it has been attempted to reduce the bitline coupling by twisting the bitlines at predetermined positions. Furthermore, it has been tried to improve the retention time of the DRAM cells by increasing the capacitance of the storage capacitor.
However, these measures have not given satisfactory results.
European Patent Application No. EP 0 730 298 A discloses a method of forming a bitline and a bitline contact. According to the method, after depositing a dielectric layer on the non-planarized surface of a DRAM memory cell array and forming a bitline contact, a wiring layer is formed. Subsequently, the second dielectric material is deposited and treated by a chemical mechanical polishing process to form a planarized surface. The second dielectric material is a low-k material, and the bitline contact is made of phosphorous-doped polysilicon.
In addition, International Publication Number WO 01/26139, which has a common assignee as the instant application, discloses a method of forming a bitline and a bitline contact. According to the method, the electrical contact between the bitlines and the source or drain portion of the DRAM cell is accomplished by first contact plugs of a conductive material, preferably polysilicon, and, additionally, by second contact plugs which are formed concurrently with bitlines in a dual damascene process. For forming-the first contact plugs, a dielectric layer is deposited over the gate structures establishing the non-planarized surface of the DRAM cell array. Thereafter, openings are formed between the gate structures and the openings are filled with a conductive material.
In U.S. Pat. No. 5,712,201 to Lee et al., tungsten plugs extending to the source or drain portions of the DRAM cell are provided. Subsequently, a metal layer of aluminum having a low copper content is deposited and patterned to form bitline contacts.
According to German Published, Non-Prosecuted Patent Application DE 199 48 571 A, which has a common assignee as the instant application, the bitline contacts are formed in an insulating material surrounding the gate electrodes, the gate electrodes establishing a non-planarized surface of the DRAM cell array. Thereafter, the bitlines are formed by depositing and correspondingly patterning an aluminum layer. The second dielectric is deposited after forming the bitline structure.
U.S. Pat. No. 5,846,881 issued to Sandhu, et al. relates to a metallization process in which TiSi is deposited by Chemical Vapor Deposition. In particular, this document discloses a method of forming a bitline contact of TiSi and a bitline of Al or an Al/Cu alloy. According to this embodiment, the bitline material is sputtered as a layer and subsequently patterned to form the bitlines. According to a further embodiment, the bitline contacts, as well as the bitlines (both made of TiSi), are formed by a dual damascene process.
It is accordingly an object of the invention to provide a method of forming a bitline and a bitline contact, and a dynamic memory cell including a bitline and bitline made contact according to the method that overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type and that provide a method of forming a bitline and a bitline contact having reduced bitline coupling between neighboring bitlines. Moreover, it is an object of the present invention to provide a dynamic random access memory cell array with reduced bitline coupling.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of forming a bitline and a bitline contact for a dynamic memory cell array. The method includes the following steps. The first step is providing the dynamic memory cell array including a plurality of memory cells formed in juxtaposition on a semiconductor substrate and being insulated from each other by an insulating material; each of the memory cells includes a storage capacitor as well as a transistor having a source, a drain, and a gate portion; the memory cell array has a planarized surface. The next step is depositing a first dielectric layer on the planarized surface of the memory cell array. The subsequent step is defining a contact hole in the first dielectric layer penetrating through the first dielectric layer. The next step is filling the contact hole with a first conductive material so as to form a bitline contact; the first conductive material electrically contacts one of the source and drain portions of the transistor. The next step is depositing a second dielectric layer on a surface of the first dielectric layer. Thereafter, the next step is defining a wiring structure of a second conductive material having a higher conductivity than the first conductive material in the second dielectric layer to form the bitline.
With the objects of the invention in view, there is also provided a dynamic memory cell array having at least one bitline. The dynamic memory cell array includes a plurality of memory cells juxtaposed on a semiconductor substrate and being insulated from each other by an insulating material therebetween. Each of the memory cells includes a storage capacitor as well as a transistor having a source, a drain, and a gate portion. The bitline electrically contacts the source or drain portion of at least one of the memory cells via at least one bitline contact. The bitline contact includes a first dielectric layer deposited on the planarized surface of the memory cell array. At least one contact hole in the first dielectric layer entirely penetrates the first dielectric layer and is filled with a first conductive material. The first conductive material is in electrical contact with the source or drain portion of the at least one memory cell, so as to form the bitline contact, a second dielectric layer on the surface of the first dielectric layer, and a wiring structure of a second conductive material having a higher conductivity than the first conductive material in the second dielectric layer.
The present invention is based on the knowledge that bitline coupling depends in a first approximation on the product of resistance of the wiring and the coupling (capacitance) thereof. At a given bitline material, the resistance thereof could be lowered by raising the height of the conductive stack. This measure in turn will increase the coupling to the opposing line.
Accordingly, introducing a new material for the bitline wiring will reduce the bitline coupling when the new material has a lower resistivity than tungsten, which is commonly used as the bitline contact material. In particular, copper has a resistivity that is approximately one tenth of tungsten. Additionally, the bitline coupling can further be reduced by introducing a dielectric having a low dielectric constant (so-called low-k dielectric).
However, previously two major reasons prevented these measures.
On one hand, in the conventionally employed dual damascene process the bitline contact material and the bitline material are deposited in one single process step and they are made of one single material. However, especially copper or aluminum cannot be employed as this single material because problems concerning diffusion will occur when copper or aluminum is deposited in contact with silicon.
On the other hand, low resistivity materials such as copper and aluminum as well as low-k materials do not withstand the retention anneal process which has to be performed as mentioned above. In particular, copper as well as low-k materials become thermally instable at temperatures above 500xc2x0 C.
As the inventor of the present invention discovered, the above problems can be handled if the dual damascene process is replaced with a two-step process in which, first, the bitline contact is established in a first dielectric layer and, then, the bitline is defined in a second dielectric layer, the bitline material having a lower resistivity than the bitline contact material.
To this end, the first dielectric layer is deposited on the planarized surface of the DRAM cell array. The planarized surface of the DRAM cell array can be established by wordlines 6 that are surrounded by spacers 17 and having an insulating material such as a BPSG layer 3 therebetween, as is shown in FIG. 1A. In this case, when defining a contact hole entirely penetrating the first dielectric layer, subsequently, also the BPSG layer 3 has to be etched in order to achieve electrical contact to the source or drain portion.
In addition, forming contact plugs filled with a conductive material are formed between the wordlines is within the scope of the invention. For example, contact plugs can be formed in every other position. In this case, when defining a contact hole, only the first dielectric layer has to be etched. After filling the contact hole with a conductive material, electrical contact to the source or drain portion is established.
According to the present invention, the first and second dielectric layers can be made of the same dielectric such as silicon dioxide deposited with the TEOS process. Nevertheless, it is preferred that the second dielectric layers has a smaller dielectric constant than the first dielectric layer so as to further reduce bitline coupling. In particular, it is preferred that the second dielectric layer is made of a so-called low-k material.
In case the method of forming a bitline and a bitline contact of the present invention is to be incorporated in the presently used standard DRAM process, it is further preferred that the retention anneal process is performed before employing a temperature sensitive material. In particular, if a low-k dielectric is used as the second dielectric layer, this retention anneal process is performed after filling the contact holes with the first conductive material and before depositing the second dielectric layer on the surface of the first dielectric layer. In this case, the second dielectric layer as well as the second conductive material is deposited after this high temperature step and the stability of these materials will not be affected. On the other hand, if the material used as the second dielectric layer withstands the temperatures prevailing during the retention anneal process, this process can also be performed after depositing the second dielectric layer and before defining the bitlines.
However, depending on the specific DRAM process used, this step can also be performed at another time, can be possibly completely dispensed with or can be modified so as not to affect the temperature sensitive material employed.
Accordingly, the present invention, in which the step of providing the bitline contact material is separated from the step of providing the bitline material, provides the advantage that the resistance of the bitlines can remarkably be reduced because the bitlines are made of a conductive material having a lower resistivity than that of the bitline contact. Thereby, the bitline coupling can be reduced. If the dielectric between the bitlines is additionally made of a low-k dielectric, the bitline coupling can further be remarkably reduced.
According to a preferred embodiment of the present invention, in which the retention anneal process is performed before depositing the conductive material or even before applying the low-k dielectric, there is no high temperature step to be performed after depositing these materials so that the reduced temperature stability of these materials will not involve any problem.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method of forming a bitline and a bitline contact, and dynamic memory cell including a bitline and bitline made contact according to the method, it is nevertheless not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.